1. Field of the Invention
The present invention relates to a plasma generating apparatus for generating Low-pressure, Low-temperature plasmas in a vacuum vessel, particularly, to the apparatus which is used for surface treatment, etching, ashing, cleaning, depositing a thin film and so on on a semiconductor substrate, a liquid crystal substrate, an organic material, a metallic material, or the like by using the generated plasma.
2. Discussion of Background
Hereinbelow, a conventional and representative plasma generating apparatus using a microwave, which can generate plasma by an electromagnetic energy of a surface wave exited and transmitted at the boundary between the plasma and a surface of dielectric material used for introducing the microwave into a vacuum vessel, will be described.
In FIG. 8, there is shown a schematical structure of a surface wave plasma generating apparatus, for example, which is described in Japanese Journal of Applied Physics, Vol. 35, (1996), p.L341-L344. In FIG. 8, numeral 1 designates a vacuum vessel in which a gas supply port 1a and an evacuation port 1b are respectively formed in the side portions thereof. Numeral 2 designates a microwave oscillator for oscillating microwave; numeral 3 designates a power source for driving the microwave oscillator 2; numeral 4 designates a dielectric plate made of a dielectric material through which microwave is permeable, which plate is disposed in a side wall of the vacuum vessel 1 and separates the vacuum atmosphere from the atmosphere; and numeral 5 designates a waveguide for guiding the microwave oscillated in the microwave oscillator 2 to the dielectric plate 4, which connects the dielectric plate 4 to the microwave oscillator 2. Numeral 6 designates a slit provided in the bottom surface of the microwave guide 5, which is in contact with the dielectric plate 4; and numeral 7 designates a substrate to be treated which is subjected to various surface treatment using the plasma generating apparatus.
The operation of the conventional surface wave plasma generating apparatus constituted as in the above is described. The inside of vacuum vessel 1 was evacuated to obtain a high vacuum by a high vacuum pump (now shown) such as a roughing pump and a turbo-molecule-pump. A gas for discharging electrons (discharge gas), for example, argon, hydrogen, oxygen, chlorine, carbon tetrafluoride, and silane, was supplied through the gas supply port la until the inside of vacuum vessel 1 has a predetermined pressure by the gas. A microwave oscillated by the microwave oscillator 2 was introduced through the waveguide 5 and emitted from the slit 6 provided in the bottom surface of the waveguide 5. The emitted microwave was introduced into the vacuum vessel 1 through the dielectric plate 4, whereby plasma 8 of the discharge gas was generated in the vacuum vessel.
It is known that when once the plasma 8 was generated, the microwave introduced into the vacuum vessel 1 became a surface wave which can transmit along with only the boundary between the dielectric plate 4 and the plasma, and that even in a case of high density plasma 8 having an electron density therein exceeding a so-called cut-off density (for example, about 7.times.10.sup.10 cm.sup.-3 in case of a microwave having a frequency of 2.45 GHz), the exited surface wave was transmitted and absorbed without being reflected. Thus, an electron of the plasma 8 in the vicinity of the dielectric plate 4 was accelerated by a vibrational electric field of the surface wave to thereby change in a high-energy-state. A neutral gas particle in a form of atom or in a form of molecule was exited, dissociated and ionized by the high-energy electron to maintain the generation of plasma 8. Therefore, it was convenient for the generation of the plasma of a high density exceeding the cut off density to couple the plasma 8 with the microwave power to be supplied in the form of surface wave.
However, the conventional surface wave plasma generating apparatus has problems that the electric field strength of microwave in the surface area of the flat dielectric plate 4; and the distribution of microwave electric field strength is in the direction of diameter and the direction of circumference of the surface area of the dielectric plate 4 depending on conditions of generating plasma, such as a gas pressure and an electric power of microwave. As the result of inhomogeneous electric field strength, the distribution of the density of electrons and ions existing in the generated plasma 8 in the diameter direction and the circumference direction in the vacuum vessel 1 became inhomogeneous.
Further, the electrons and the ions in the generated plasma 8 inject into the inner surface of vacuum vessel 1 or the inner surface of the dielectric plate 4, and thereafter extinct immediately by a combination process. Therefore, in order to maintain the generation of the plasma 8, ionization, namely the generation of plasma, sufficient to compensate the loss of plasma caused by the recombination in the wall surface was necessary. However, in the conventional plasma generating apparatus, the rate of loss of plasma in the wall surface was large because an effect of confining electrons and ions did not exist. Accordingly, in the conventional surface wave plasma generating apparatus, there were problems that the plasma was hard to catch fire; the plasma was difficult to be generated; and the maintenance of generation of the plasma was difficult.